As typified by a drop in the price of the plasma television, the screen of television receivers for ordinary households is becoming greater. For the diversity of television receivers using a large screen such as a wide screen in particular, there are many plasma television receivers which have a function that the screen is split into two parts to watch different programs (contents) at the same time. At this time, it is easy to watch the desired contents of pictures shown on the screen in multiple windows. However, when sounds are normally outputted at the same time, it is almost impossible to select only the sounds desired to hear. On this account, in the television receiver having the function to split the screen into two parts, sounds of first contents are outputted from speakers, and sounds of second contents are outputted from earphones. However, in the case of the television receiver like this, a viewer who watches the second contents has to use earphones all the time, causing a problem that usability is not good.
Then, an audio output apparatus is proposed in which directivity is controlled to output sounds in such a way that a plurality of sounds is provided with different directivities respectively (for example, see Patent Reference 1). In this audio output apparatus, it is proposed to use an array speaker for control over sound directivity. The array speaker has an advantage that has excellent control over directivity and can output a plurality of sounds in different directivities at the same time. In recent years, base technologies for the array speaker such as a digital amplifier and a small-sized full range speaker are progressing, and a digital processing circuit for delay and signal processing is reducing in price. Therefore, it is effective to use the array speaker for control over sound directivity.
Here, the principle of an array speaker in a known delay array system which is effective for directivity control and known for a long time will be described with reference to FIG. 13. It is considered that a large number of small-sized speakers 201-1 to 201-n are arranged linearly, and assuming that a straight line connecting a focal point P to each of the speakers 201-1 to 201-n is extended, an arc Z is defined so that the distance from the focal point P is L, virtual speakers 202-1 to 202-n indicated by broken lines shown in FIG. 13 are arranged on the intersection points of the extended straight lines with the arc Z. Since the distance from the virtual speakers 202-1 to 202-n to the focal point P is all L, sounds emitted from each of the speakers 202-1 to 202-n reach the focal point P at the same time.
In order to reach the sounds emitted from an actual speaker 201-i (i=1, 2 n) to the focal point P at the same time, a delay (time difference) corresponding to the distance between the speaker 201-i and the corresponding virtual speaker 202-i may be added to the sounds outputted from the speaker 201-i. More specifically, when seen from the focal point P, the virtual speakers 202-1 to 202-n are controlled as though they are arranged on the arc Z. Accordingly, at the focal point P, the output phases of the individual speakers 201-1 to 201-n are aligned, and a peak of sound pressure is formed. Consequently, directivity distribution can be obtained as though a sound wave beam is emitted toward the focal point P.
Patent Reference 1: JP-A-H11-027604
As described above, in the array speaker in the delay array system, it has an advantage that only the delay time is changed to move the orientation of sounds freely and a plurality of sounds can be outputted in different directivities at the same time. However, in the case of the audio output apparatus of Patent Reference 1 using the array speaker like this, it has some points to be problems for practical use.
FIG. 14 shows an exemplary polar pattern in simulation. This simulation is the case in which the focal point is formed at the front by a practical array speaker which is linearly arranged to have the total array width of about one meter. The front is the upper side in FIG. 14. According to the exemplary simulation shown in FIG. 14, it is revealed that with respect to a high audibility sound of 2 kHz, a sound pressure difference of 20 dB can be realized at the position two meters distant from the array speaker in the direction at an angle of 30 degrees distant from the central direction of directivity. This sound pressure difference allows a viewer in the central direction of directivity to hear the sounds of contents at the moderate sound level and another viewer at the different position to hear the sounds of the same contents at a low sound level. When a plurality of sounds is outputted in different directivities as the audio output apparatus of Patent Reference 1, for individual viewers, they hear mixed sounds of the sounds of contents that the viewers desire to hear at the moderate sound level with the sounds of the other contents at the low sound level (disturbance sound).
The importance in the audio output apparatus of Patent Reference 1 is that the sounds of the other contents become the sound level smaller enough than the sounds of contents that are desired to hear. When there is a sound pressure difference like this, the masking effect that is the characteristic of audibility and the cocktail party effect that is the characteristic of psycho acoustics serve in a manner to aid hearing target sounds. Therefore, the viewer can hear the target sounds of contents among a plurality of sounds.
However, when the absolute sound levels of a plurality of sound signals inputted to the array speaker are greatly varied, the sound pressure difference realized by directivity control over the array speaker is likely to be cancelled. When the sound pressure difference between the target contents and the other contents becomes insufficient, the sounds of the other contents become annoying, and at the worst, the target sounds of contents cannot be heard. For the reason why the sound pressure difference between the target contents and the other contents become insufficient, two reasons can be mainly considered.
The first reason is that the recording levels of sounds are varied at each of the contents. Since it is natural that the recording levels of sounds are varied at each of the contents, a volume control of the audio output apparatus sets the sound level of each of the contents to the optimum value (the value that the separation of audibility becomes optimum at the positions of the individual viewers). However, even though the volume control is set to the optimum value during the playback of certain contents, this setting for the volume control might not be suitable in the playback of the other contents. When the volume control is set unsuitably in this manner, the sound pressure difference of the contents different from the target contents becomes insufficient, and the separation of audibility is deteriorated. In order to improve hearing the target sounds of contents, the volume control needs to be adjusted for each of the contents.
The second reason is that the sound level of contents is changed at any time. For example, when a sound such as an explosion sound is played at a high sound level in different contents while a silent section is continued in target contents, this change in the sound level reverses the sound pressures of the target contents and the other contents.
In addition, for another problem that affects the separation of audibility, there is a problem that it is difficult to perform directivity control over a wide sound frequency band. When a delay array is taken as an example, the main lobe width of directivity is determined by the ratio of a signal wavelength to the width of an array speaker. High audio frequencies have strong directivity, whereas low audio frequencies have weak directivity. With reference to FIG. 14, it is revealed that directivity is changed by frequencies. The directivity becomes weak in the low audio frequencies, and it is difficult to secure the separation. On the other hand, in high audio frequencies that are wavelengths shorter than the pitch between speaker units of the delay array, a grating lobe is generated in a directivity pattern, and a side lobe is generated in the directivity pattern even in the wavelengths longer than those frequencies. Therefore, the grating lobe and the side lobe might deteriorate the separation of audibility.